Development of a flame transfer function framework for transversely forced flames

This paper describes a framework for the development of a flame transfer function for transversely forced flames. While extensive flame transfer function measurements have been made for longitudinally forced flames, the disturbance field characteristics governing the flame response of a transversely forced flame are different enough to warrant separate investigation. In this work, we draw upon previous investigations of the flame disturbance pathways in a transversely forced flame to describe the underlying mechanisms that govern the behavior of the flame transfer function. Previous transverse forcing studies have shown that acoustic coupling in the nozzle region can result in both transverse and longitudinal acoustic fluctuations at the flame, and that the acoustic coupling is a function of combustor geometry, and hence, frequency. The results presented here quantify this coupling across a large range of frequencies using a velocity transfer function, F_TL. The shape of the velocity transfer function gain indicates that there is strong acoustic coupling between the main combustor section and the nozzle at certain frequencies. Next, measured flame transfer functions are compared with results from theory. These theoretical results are derived from two level-set models of flame response to velocity disturbance fields, where velocity inputs are derived from experimental results. Data at several test conditions are presented and larger implications of this research are described with respect to gas turbine combustor design.